A relay is a control device that allows a small voltage or current to control a larger voltage or current. Use this tag for questions about standalone relays in electrical or HVAC wiring -- questions about relays in appliances do not need this tag.
An electromechanical relay consists of a solenoid coil mechanically linked to one or more sets of switch contacts. They may run off line or low-voltage AC or low-voltage DC, and are available in a variety of contact forms and terminations. Solid-state relays use an optical link between a LED and an optically-controlled solid-state switching device such as a transistor or triac instead of a magnetic coil and electrical contacts, and may be found in some applications, particularly when loads are switched frequently.
Relays are commonly found in HVAC systems to interface the low-voltage (24V) control wiring from the thermostat to high-power loads such as fans, heater elements, and compressors. These relays may also be packaged together with a small control transformer in a single enclosure, and are commonly used in ad-hoc HVAC control applications.
They are also used in electrical applications to provide remotely or automatically-controlled switching functions, particularly for lighting. Such relays must be UL listed for these applications, and are packaged for convenient connection to electrical wiring systems.
Appliances and control circuit boards also contain relays for similar purposes, but this tag is not needed when discussing broken control boards, as component-level control board troubleshooting is generally not done in the context of this Stack.